1. Field of the Invention
The invention relates to movable switch points for railway switches and, more particularly, to devices for compensating for lost motion between the throw of the switch machine and the movement of the switch point.
2. Description of the Prior Art
A "frog" is the location at which one rail crosses over or intersects another rail. In instances of high speed turnouts (i.e., where a railway vehicle switches from one track onto another track), the actual degree of switch or turnout may be very long because at higher speeds it is desirable that the train make the transition from one track to the other at a slower rate. Because of the long length of turnout, means have been devised in which to separate the rails.
One means to separate the rails is to make the frog section of the track a movable point. Thus, the frog which lies between and separates two sections of rail is connected to a means for moving the frog called a switch machine. An operating rod (also referred to as a "throw rod") is connected to and caused to be translated by the switch machine. The switch machine and operating rod, together with a second operating rod and a switch point adjuster (as described in more detail below), cause the switch point to move.
The distance by which the frog must be moved (i.e., the "throw") is typically between two inches and five inches. However, switch machines per AAR (Association of American Railroads) recommendations and standards, will always throw six inches, regardless of the type of switch machine utilized. Therefore, if the switch machine throw is six inches and is connected to the frog through a rigid connection, the frog also must be moved six inches. However, if the frog is only to be moved somewhere between two to five inches, a means must be used to compensate for the lost motion of the switch machine. For this reason, the switch machine is connected through an operating rod to a switch point adjuster.
A switch point adjuster is a device that compensates for switch machine operating lost motion and maintains switch point pressure on the frog or switch point as a train travels through. The switch point adjuster takes up the lost motion between the switch machine throw and the switch point displacement. This is done by allowing the switch operating rod to move a given distance before making contact with the opposite end of the switch point adjuster. Only after this given distance of travel does the machine begin to drive the switch points. Pressure between the extended sleeves of the operating rod and the switch point adjuster is present on one side of the adjuster--the side keeping the switch point closed. By adjusting the sleeves on the threaded operating rod, the point opening can therefore be adjusted to ensure that the point is closed and has adequate pressure on it when the train travels over the rail switch.
Referring to FIG. 1, a prior art switch point adjuster 2 is schematically depicted. As can be seen, the prior art switch point adjuster 2 utilizes two separate rods 3, 4. Two separate rods are used because maintenance personnel were unable to easily access the bottom of the switch point 16, therefore, there was no way of easily making any adjustments to the switch point adjuster 2 right at the point, as the track 14 itself would prevent access to the switch point adjuster 2. Thus, the switch point adjuster 2 was located at the center of the track 14 where maintenance personnel could access it. In order to do that, a two rod configuration was utilized: a first rod 3 connects the switch point adjuster 2 to the frog and a second rod 4 connects the switch point adjuster 2 to the switch machine 12. Thus, when the switch machine 12 throws six inches, the slack is taken up in the switch point adjuster 2 so that the frog is only moved its required amount. Both operating rods 3, 4 are supported by support rollers.
There are several drawbacks associated with this prior configuration. For example, if there is a problem with either of the operating rods, the amount of throw at the switch point may vary. Also, the flexure or lateral movement of both rods must be accounted for in designing the switch point adjuster. Furthermore, adjustments made to the switch point adjuster are more difficult when two operating rods have to be adjusted.